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MX Record Answer Contains Same Domain




What does it mean for an MX record to have an answer section that contains itself? My earlier belief was that this implies that a domain is it's own mail domain, but from running a couple experiments on web domains, I get connection timeouts when doing SMTP scans on domains that have MX records as below.



Which RFC / where in an RFC contains this specification?



$ dig -t mx yahoo.net                                                                                                                                                

; <<>> DiG 9.11.3-1ubuntu1.7-Ubuntu <<>> -t mx yahoo.net                                                                                                                                           
;; global options: +cmd                                                                                                                                                                            
;; Got answer:                                                                                                                                                                                     
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 29654                                                                                                                                          

;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1                                                                                                                               

;; OPT PSEUDOSECTION:                                                                                                                                                                              
; EDNS: version: 0, flags:; udp: 65494
;; QUESTION SECTION:
;yahoo.net.                     IN      MX

;; ANSWER SECTION:
yahoo.net.              1800    IN      MX      0 .


;; Query time: 175 msec
;; SERVER: 127.0.0.53#53(127.0.0.53)
;; WHEN: [REDACTED] UTC 2019
;; MSG SIZE  rcvd: 53


Other domains that have MX records like this:
umblr.com, google.az, ardmediathek.de, huffingtonpost.ca, yimg.com, healthdirect.gov.au


Answer



First off, as a general observation, the name . does not refer to "the domain itself" but to the root of the DNS tree.




Specifically in the context of MX, though, the rdata 0 . has been defined to mean "null", as in "I do not want mail delivered".



No MX, on the other hand, has a completely different meaning; try delivering to the address directly associated with this name instead (in practice this is often some web server).


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